Egg activation is the result of calcium signal summation in the mouse

Egg activation in mammals is caused by cytosolic Ca(2+) oscillations that are essential for development. However, despite increasing knowledge about signal transduction mechanisms, the functional linkage between frequency number, amplitude and duration of the Ca(2+) signal and the kinetics of pronucleus formation has not yet been defined. While a wide range of Ca(2+) signal parameters are efficient in causing egg activation, the basic rules governing how the egg integrates these signalling events are not yet clear. Thus, in the perspective of better understanding how the egg processes Ca(2+) signalling events, the objective of this study was to determine experimentally whether the efficiency of egg activation and the subsequent early developmental stages rely on Ca(2+) signalling summation. Non-fertilized, but freshly ovulated mouse eggs, were subjected to a series of repetitive Ca(2+) influxes of various patterns modulated by a non-invasive membrane electropermeabilization method. Using a combination of two suboptimal treatments we have shown that mouse eggs can sum up the effects caused by various patterns of intracellular Ca(2+) concentrations transient during the period of egg activation. In addition, overloading the intracellular milieu by repetitive Ca(2+) influxes did not seem to inhibit the process of activation. The kinetics of pronuclear formation among a population of eggs treated in the same conditions became accelerated when the total dose of Ca(2+) signal 'experienced' by the eggs was increased. The results suggested that summation of the biological effects of all Ca(2+) signals constitutes an important mode of Ca(2+) signal integration.